Reduction of Carbon-Carbon Double Bonds by Hydrazine

A study was made of the mechanism of reduction of olefinic bonds by hydrazine. It was established that reduction takes place only in presence of oxygen (or a suitable oxidising agent) and that two molecules of hydrazine are required to hydrogenate
one double bond. The overall stoichiometry of the reaction was found to be represented by the following equation:
-CH=CH- + 2N2H4 +3/2 02 —> -CH2-CH2- +2N2 + 3 H2O
Investigations of the effects on the reaction of the appropriate variables shown it to occur only in presence of weak acids, though under alkaline conditions, and in ionizing media. These results, supported by measurements of conductance changes during the reaction, indicate that the hydrazonium ion, and not the hydrazine molecule, is the initial reactant. The above facts, together with detailed studies of reaction kinetics, led to the suggestion of the following reaction mechanism. It is postulated that the overall reaction consists of three main stages:
(i) oxidation of hydrazonium ion to protonated diimide (diimide cation):
(ii) less of the proton by the diimide cation under the action of hydroxyl ions with the formation of an equimolar mixture of syn- and anti- diimide;
(iii) hydrogenation of the double bond by the syn- diimide via the formation of a cyclic, hydrogen-bonded complex.

The anti- diimide is not capable of reducing the olefinic link and it is further oxidised to nitrogen and water.
The experimental work on the mechanism of reaction was performed with oleic acid and oleyl alcohol. However, reduction by hydrazine has also been successfully applied to a variety of other unsaturated compounds, including acids, alcohols, hydrocarbons, carbonyl and ring compounds. Hydrogenation of the double bonds always occurred provided there was no interference from side reactions with hydrazine.